Question

Which of the following is most effective towards coagulation of CdS sol?

• A. \(K_{2}SO_{4}\)
• B. \(CaCl_{2}\)
• C. \(Na_{3}PO_{4}\)
• D. \(AlCl_{3}\)

Hint:

For negatively charged sols, cations are responsible for coagulation. Always compare the valency of the cations to determine the strongest coagulant.

Answer 1

The Correct Option is D

Step 1: Recall the Hardy-Schulze rule.
To coagulate a sol, the ion with charge opposite to the sol does the work. The higher its charge, the stronger it coagulates.
Step 2: Find the sol charge.
CdS sol is negatively charged, so we need positive ions (cations) to neutralise it.
Step 3: List the cations.
The choices give $K^+$, $Ca^{2+}$, $Na^+$ and $Al^{3+}$.
Step 4: Compare their charges.
Coagulating power follows charge: $Al^{3+}>Ca^{2+}>Na^+=K^+$.
Step 5: Pick the strongest.
The $+3$ aluminium ion is the most powerful, so $AlCl_3$ is the best coagulant.
Step 6: State the answer.
\[ \boxed{AlCl_3} \]